Page 224 - Advances in Eco-Fuels for a Sustainable Environment

188 Advances in Eco-Fuels for a Sustainable Environment

encourage employment and economic development in rural areas, to develop a long-
term replacement for fossil fuels, to reduce national dependency on petroleum
imports, and to increase the security of the energy supply.
The development of first-generation biofuels from food and oil crops-including
sugarcane, sugarcane beet, rapeseed oil, vegetable oil, and animal fat—has attained
stable economic production with certain limitations [3]. The first-generation biofuels
raised a major concern on food security as the majority of oil was extracted from edi-
ble food crops [4]. The advent of second-generation biofuel has created a space for
biofuels as the fuels were obtained from agricultural and forest residues as well as
wood processing waste [5, 6]. The waste cooking oil is also considered to be an eco-
nomical source for the production of biofuels as it is cheaper than the other sources [7].
The selection of feedstock should be economically and technically viable such that it
should require low or no land usage with the emission of good quality air. The
nonedible oil offers another promising source for the production of biofuels as it over-
comes the problems associated with the use of edible sources [8].
Microalgae are regarded as third-generation biofuels and are considered a poten-
tially inexhaustible sources. This potential is because of their availability and the pres-
ence of the highest lipid content in them. Microalgae are considered to be one of the
most hopeful sources for biodiesel production. Algae are divided into two major
groups based on their size and morphology, as macroalgae and microalgae based
on their thallus size [9]. Microalgae are prevalent both in fresh and marine water
[10]. Microalgae are photosynthetic autotrophs as well as mixotrophic and heterotro-
phic microscopic organisms. Both microalgae (autotrophs and heterotrophs) were
found to have different fractions of biodiesel yield [11]. Microalgae are ecofriendly,
they demand less area to grow, and they are rich in oil. Microalgae are the only organ-
isms known so far that are capable of both oxygenic photosynthesis and hydrogen pro-
duction. Microalgae can be grown in lab scale as well as in natural open ponds. Wild
algae, that is, natural inhabitants are required for this purpose. Microalgae have simple
growth requirements as they use light, carbon dioxide, and other inorganic nutrients
efficiently and are capable of growing in diverse environments [12].
Microalgae have an additional ability of mitigating the CO 2 level in the atmo-
spheric pollution. It is estimated that 1.8kg of CO 2 is required for producing 1kg
of algal biomass [13]. The oil content of microalgae is found to be more than 80%
of its dry weight, some of the algae have about 15%–40% (dry weight). In comparison,
the oil content of a palm kernel is about 50%, copra has 60%, and sunflower contains
55%. In fact, microalgae have the highest oil yield among various plant oils.
Therefore, microalgae were identified as promising feedstock due to their high bio-
mass productivity and lipid accumulation. The priority of macroalgae for bioenergy
production is given due to (i) higher photon conversion efficiency, enabling rapid syn-
thesis of biomass ([14]); (ii) a higher photosynthetic rate ([15]); and (iii) they do not
require arable land for growth [16, 17]. The microalgal bio-oil has shown to have high
heating value, low density, and low viscosity as compared to the bio-oil obtained from
other sources (wood). The main challenges in the development of algal biofuels
include strain selection and extraction with higher biomass yield [9, 11, 18]. Bio-
oil is mainly produced by physical or chemical conversion or by thermochemical